I say go with the flow! If the time feels right then you should do whatever seems to need doing. As things start to sprout, cut off the dead, the plant will be ready to grow. If the weather turns now it can't be that bad, not to hard frosts anyway.
Your Cannas are most likely not tall growing types, or does your sister-in-law have the same ones? Cannas do like to be moist but not waterlogged in cold weather, there are some water varieties. Some good compost will certainly help them.
C. musifolia is a tall grower, the cultivar Grande has red edges, I have got that this year and it should be hardy, I will take some off as insurance (says she who left her Ensete ventricosum in the ground to go to mush!) . C. altensteinii I grew from seed is new from Africa, it got fairly tall last year, at least 7 feet in it's second year, but it's supposed to be tender and in the right conditions can grow to 15'. I took some new shoots off, but left the main plant outside in a pot, it was sending up a new shoot until the hard frost, I think it will grow still but it took -5C with no bother.
I have a lot of Hemerocallis in pots that were romping away too until the hard frost, they now look yellow but will recover, but for one frost many plants would have been months ahead now!
My Melianthus has more than one stem, it sends up shoots so when yours has finished flowering you could perhaps take some off with roots if you have any. The leaves smell of peanut butter, they say it's unpleasant but I find it only slightly bitter smelling, quite a surprise when you brush past it. What a surprise having flowers though!
It is spring again today, so lovely, I pulled a few weeds while walking around the garden, the sun on my back was heavenly! I have to do the shopping today, darn!